SH% Dwell Time
+ Mix Recipe + Cost

Divide the target surface SH% by your stock SH% to get the fraction of stock in the mix. For 1% target from 12.5% stock: 1 ÷ 12.5 = 0.08, so 8% of your 100-gallon mix is SH stock — that's 8 gallons. Fill the rest with water plus 1 gallon of surfactant. This calculator does the math automatically for any combination.

Sodium hypochlorite evaporates faster in heat. At 90°F+ the SH off-gasses 25–35% faster than at 75°F, so you get less contact time before the surface dries — which causes flash-drying and white streaks. Hot surfaces also break down the surfactant film faster. Below 65°F the chemistry slows and needs 15–30% longer dwell to achieve the same kill rate on organic growth. This calculator adjusts the recommended dwell window automatically based on your input temperature.

Asphalt shingles typically need 3–6% SH on the surface to kill gloeocapsa magma (black streak algae) — which means using higher stock concentrations or less dilution. Vinyl siding needs only 0.5–2% SH on surface to clean mold and algae, and higher concentrations risk accelerating UV oxidation. Painted aluminum and composite decks should stay at or below 1% on surface. This calculator shows the substrate-specific maximum and flags if your target exceeds it.

Field standard is a minimum 3-minute contact time on any surface. Less than 3 minutes doesn't provide sufficient kill time on mold, mildew, and algae. For lichen and moss you often need 10–15+ minutes or a re-application. The maximum safe dwell on painted or coated surfaces (vinyl, aluminum, composite) is 15 minutes — beyond that you risk oxidation, paint lift, or manufacturer warranty voids.

The cost depends primarily on how much SH stock you use. At $3.75/gallon for SH stock and $0.60/gallon for surfactant (1 gallon per 100-gallon mix standard), a 1% target mix from 12.5% stock (8 gallons SH) costs roughly $30–32 per 100 gallons including surfactant. A 3% target mix (24 gallons SH) costs roughly $90–95. This calculator shows the exact cost for your inputs at the default unit prices.